1. Field of the Invention
The present invention relates to devices which selectively permit or block the passage of light therethrough by reversing the polarity of a voltage which is applied across a bistable ferroelectric material, and in particular, to ferroelectric liquid crystal devices capable of producing images having superior contrast and which can be changed quickly between light transmissive and light blocking states.
2. Description of Related Art
Light shutters are devices which can be controlled to selectively block or permit the transmission of light therethrough. Light shutters have numerous applications. For example, a single light shutter can be used in systems which transmit data optically to permit or prevent the transmission of optical signals therethrough much like an electrical switching device is used in systems which transmit data electrically. A linear array, or a matrix of light shutters can be arranged between a light source and a photosensitive material such as, for example, a photoconductive drum or belt, in an image producing machine such as a copier, printer, or facsimile machine. As the photosensitive material moves past the array or matrix of light shutters, the light shutters are selectively actuated to block or permit the transmission of light from the light source to the photosensitive material to form a latent image on the photosensitive material. This latent image is, for example, toner developed and then transferred to a sheet of paper to form a permanent image on the sheet. A matrix of light shutters is also typically used to form a display or display screen wherein the light shutters are selectively actuated to form images on the display screen by controlling the transmission of light through portions of the display screen or by controlling the reflection of light by a surface located behind the display screen. Other uses of light shutters are known and possible and are intended to be covered by the present invention. For example, it is known to use liquid crystal display devices for copiers, printers, or the like. See, for example, Xerox Corp. U.S. Pat. Nos. 4,506,956, 4,527,864, and 4,475,806.
Liquid crystals are commonly used to form light shutters. Liquid crystals are well known and, generally, are made from materials which exhibit more than one refractive index depending upon their orientation, and whose orientation can be changed by the application of an electrical potential. It is also known to use ferroelectric materials to form liquid crystals. See U.S. Pat. No. 4,367,924 to Clark and Lagerwall, the disclosure of which is herein incorporated by reference. The ferroelectric liquid crystal in a suitably prepared device has bistability, i.e., has two stable states comprising a first optically stable state (first orientation state) and a second optically stable state (second orientation state), with respect to an electric field applied thereto. Accordingly, the liquid crystal is oriented to the first optically stable state in response to one electric field vector and to the second optically stable state in response to a reversed electric field vector. Further, this type of liquid crystal very quickly assumes either one of the above-mentioned two stable states in response to the direction of an electric field applied thereto and retains such state in the absence of an electric field.
Thus, ferroelectric liquid crystals are polarity sensitive. In any device in which they are used, their response depends upon the sign of the applied voltage as well as upon its magnitude. This unique feature lends itself to the construction of a class of devices in which the optical properties of the device and its speed of response can be controlled in a new and beneficial way. This new way of constructing and operating these devices imposes little added complexity to the liquid crystal technology as currently practiced.
Consider the following conditions. Liquid crystal devices (or light shutters) include a liquid crystal material (e.g., a ferroelectric material) sandwiched and sealed between first and second substrates (e.g., glass plates). In order to control the orientation of the liquid crystal material sandwiched between these two substrates (and thus the light transmission through the liquid crystal material), one or more electrodes (hereafter referred to as pixel electrodes) are formed on, for example, the first substrate. These pixel electrodes can be made, for example, from an electrically conducting, transparent material such as, for example, indium-tin-oxide (ITO). At least one counterelectrode (also referred to as a backplane electrode) is located on the second substrate and is attached to, for example, ground potential. When an appropriate voltage is applied to a pixel electrode, the liquid crystal material located between that pixel electrode and the backplane electrode is oriented to one of the light blocking or light transmitting states. Thus, in constructing these devices (and in particular, devices such as image bars or displays having a plurality of light shutters and thus a plurality of pixel electrodes), one must delineate electrode areas (which function as pixel electrodes) by physically removing conducting material from a substrate, otherwise all image defining areas (pixel electrodes) would be shorted together. This removal leaves regions between the pixel electrodes where the state of the liquid crystal is undefined. These inter-pixel regions are typically obscured with light shields otherwise the contrast of the device will suffer from uncontrolled transmission of light through the inter-pixel regions. The process of providing light shields is not a trivial one since the amount of material that can be deposited to reduce uncontrolled light transmission is constrained in these devices. Thus, it would be beneficial if the light transmission in these inter-pixel regions was actively controlled, thereby reducing the amount of light transmission substantially below that which would normally occur if the inter-pixel regions were not shielded.
Secondly, the response of a ferroelectric liquid crystal to application of an electric field consists of several stages. There is a delay period after onset of the field during which the material starts to react to the field; this delay is typically characterized as the 0/10% time. This delay period is followed by a rapid transition to the other optically stable state. By convention, the rise time of the device is defined by the time required to switch between 10% and 90% of the full transmittance between the optically stable states. A similar action occurs when the liquid crystal is energized by the opposite polarity to reverse the condition of the system. That is, there is a 100/90% delay time followed by the 90/10% fall time. The various response times are not necessarily symmetric. After application of a field, the system in time will reach an equilibrium at which the light transmission therethrough reaches a maximum of 100%, or conversely, a minimum of 0%.
In a device such as an image bar, for example, it is preferred that the material reach equilibrium for optimum contrast. Otherwise, neither the brightest "on" nor the darkest "off" state is reached. As the speed of operation is increased at constant operating voltage, the contrast will fall because the delay time becomes a significant part of the total response time. Ordinarily, the operating voltage must be increased to overcome these delays. A way of reducing delay times without increasing operating voltage would be beneficial in decreasing the overall delay time of these devices.
U.S. Pat. No. 4,846,560 to Tsuboyama et al discloses a ferroelectric liquid crystal device which controls the liquid crystal orientation between pixel electrodes. A matrix pixel structure is provided and includes a pair of substrates respectively provided with scanning electrodes and signal electrodes intersecting with each other, with a ferroelectric liquid disposed between the scanning and signal electrodes. Each intersection between a scanning and signal electrode forms a pixel. The orientation of the liquid crystal at portions other than the intersections of the signal and scanning electrodes is controlled by providing a base plate with electric charges which provide a voltage exceeding the threshold voltage across the entire ferroelectric liquid crystal so that non-pixel portions of the ferroelectric liquid crystal are maintained in a non-light transmissive state. If the base plate is external to the device, the alignment field is low unless a large charge is deposited onto it. Furthermore, if the base plate is non-conducting, the charge must be applied by a corona, or any other ion-deposition means, a cumbersome process not susceptible to an easy change in field. Still further, since the orienting field is applied everywhere, a higher field is required internally to switch the pixel to a transmitting state because of its opposite polarity. If the base plate is internal to the device, then multiple layers of electrodes are required and must be deposited onto at least one of the substrates. This greatly increases the complexity and cost of the device because of the potential for shorts. The increased switching field would still be required as in the case of an external base plate.
U.S. Pat. No. 4,834,506 to Demke et al discloses placing a mechanical mask between pixel electrodes by printing a matrix of black lines wherever electrode material is removed. See column 2, lines 4-7.
U.S. Pat. No. 4,602,850 to DeBenedetti discloses placement of a light shield behind pixel electrodes to form a barrier which blocks electrostatic coupling between the circuitry which addresses each pixel electrode and the liquid crystal material. DeBenedetti also discloses providing a biasing electrostatic field to the liquid crystal layer by using the shield as an electrode. The potential applied across the liquid crystal by the shield electrode is less than the threshold voltage of the liquid crystal material. This enables smaller incremental voltages to be applied to the individual pixel electrodes to improve the response time of the liquid crystal material.
U.S. Pat. No. 4,493,531 to Bohmer et al discloses methods and arrangements for improving the response time of twisted nematic liquid crystal material by superimposing varying and DC fields in the liquid crystal material. A twisted nematic device has a much different response characteristic than a ferroelectric liquid crystal. Consequently, a field applied to a twisted nematic device would have a different purpose and effect than one applied to a ferroelectric device. In Bohmer, a high frequency field, to which the liquid crystal does not respond, is applied to assist in the relaxation of the liquid crystal when a D-C aligning field is removed. This high frequency field interacts with surface molecules which then influence the relaxation of interior molecules. In the present invention, a transverse field is applied to a ferroelectric liquid crystal to decrease the response time. This transverse field must be in the frequency range in which the molecules everywhere react to the field, otherwise the response time is not modified.
U.S. Pat. No. 4,896,945 to Ooba et al discloses the use of mechanical masks or light shielding layers between pixels in a liquid crystal device. See column 5, lines 58-61.
U.S. Pat. No. 4,747,671 to Takahashi et al discloses a liquid crystal using a ferroelectric material wherein an electrode having a delay function is connected between a signal source and a transmission electrode on a substrate which contacts the ferroelectric material.
As further background regarding methods of driving liquid crystals and ferroelectric liquid crystals, see U.S. Re. Pat. No. 33,120 and U.S. Pat. No. 4,769,659.